Much effort has been devoted to removing pollutants such as organic liquids, synthetic dyes, and heavy metal from water in view of their detrimental impact on the environment and human health. State of the art methods to purify water include desalination, filtration, osmosis, adsorption, disinfection, and sedimentation. Amongst the various options, adsorption has many advantages, such as low costs, ease of operation, and fewer secondary products.
Adsorption refers to a process whereby pollutants are held in place on a surface of an adsorbent via physical forces such as capillary force and electrostatic force. Examples of adsorbent materials include polymer, activated carbon, silica, metal oxide, and natural sorbents such as cotton, rice husk, zeolites, and expanded perlite. After pollutants are adsorbed on the adsorbent, the physical forces holding the pollutants in place may be removed using methods such as heating and/or mechanically working the adsorbent. In so doing, the adsorbent may be regenerated and certain pollutants such as oil, toxic organics, and expensive metals may be recovered. Notwithstanding the above, problems associated with the adsorption process include poor efficiency of the adsorbent materials and/or introduction of other types of contamination during regeneration.
Carbon-based nanomaterials, in particular carbon aerogels, carbon nanotubes, graphene, and their composites, represent a promising type of adsorbent for water treatment, and have shown their great potential applications for removal of a range of synthetic dyes, heavy metals, oils, and organic solvents.
There exist two sets of adsorption mechanism for various types of pollutants. For example, electrostatic force is mainly responsible for adsorption of heavy metals and synthetic dyes, which requires that there are many charged sites on a surface of carbon-based adsorbent. On the other hand, capillary force is mainly responsible for sorption of oils and organic solvents, which require that the carbon-based adsorbents have high porosity. Unfortunately, these two types of pollutants generally coexist in wastewater, and it is difficult to find suitable carbon-based adsorbents that are able to adsorb both types of pollutants at the same time.
In view of the above, there exists a need for improved adsorbents that overcome or alleviate one or more of the above-mentioned problems.